This invention relates generally to thermal inkjet pen drive circuitry, and more particularly, to a circuit for actively warming the inkjet pen printhead to maintain a desired temperature during printing.
A conventional thermal inkjet pen includes multiple inkjet nozzles formed on a common die. Associated with each nozzle is a heating resistor and a drive transistor. The nozzle includes a nozzle chamber within which the heating resistor is located. To fire ink from the nozzle chamber the drive transistor outputs a firing pulse to the heating resistor. The firing pulse is a current pulse of sufficient magnitude to heat up the resistor enough to heat the ink to a firing temperature. The ink ejects from the chamber toward a print media sheet. The signals from respective drive transistors cause corresponding nozzles to fire. A controller circuit determines when any given nozzle is to fire. To minimize the time for the ink to heat up sufficiently to eject from the nozzle, it is known to preheat the heating resistors. The ejection temperature then is achieved a shorter time after the drive signal originates at the drive transistor.
According to a known preheating method, the temperature of the printhead is monitored during a print job. Whenever the detected temperature falls below a threshold temperature level, the controller signals the drive transistors to output a warming pulse to corresponding heating resistors. Such pulse is a current pulse having a lower magnitude than the current for a firing pulse. Specifically, the current magnitude is insufficient to raise the ink temperature enough for the ink to eject from the nozzle. One shortcoming of such approach is that the drive transistors can not perform warming operations during active printing of printhead nozzles. This becomes a shortcoming in for example a case of low density printing. When printing at a lower print density, the printhead may not stay at the desired warming temperature. Firing pulse bandwidth limitations, however, prevent sending warming pulses to the heating resistors.
Another known preheating method is to include passive resistor elements on the printhead apart from the nozzles. Signals to the passive resistors maintain the printhead at a desired threshold temperature. Such an approach, however, adds area and interconnect requirements to the printhead. Specifically more area is needed to include the passive resistor elements at frequent locations, and to provide interconnects to feed a signal to the resistor elements from the controller. In addition, the use of passive resistor elements is limited by current density limits for the printhead die. Accordingly there is need for an alternative preheating method.